The present invention relates generally to the quenching of heated coke and, more particularly, to a method of quenching heated coke to limit the amount of stress incurred by a coke drum containing the heated coke.
In a delayed coking process, the coke drum must be cooled after it is filled with hot coke to allow safe removal of the coke from the drum. Usually, water is injected into the coke drum to quench both the hot coke and the drum to a safe temperature level. In order to prevent undue stress which may cause damage to the drum, the rate at which the quench water is introduced must be controlled. A number of control methods have been used.
One method limits the quench rate to a predetermined maximum limit that will safely minimize metallurgical stresses caused by longitudinal thermal gradients in the drum. Such a method ensures a long operating life for the coke drum, regardless of the actual dynamic conditions encountered during the quenching. Usually, in this method, the quench water is introduced into the drum in a stepwise rate sequence.
Since each coke drum has unique quench characteristics for the particular coke formed in the coke drum, it is time consuming to establish a quench sequence for each batch of coke. Typically, to prevent excessive metallurgical stresses regardless of the batch of coke in the drum, the quench period is set for an extended time period. In practice, high stresses are imposed on the coke drum, because it is impossible to predict the variations in coke drum response during the quenching process. The accumulated result of periodically induced high metallurgical stresses either reduces the useable life of the coke drum or increases the maintenance repair costs.
A second quench method adjusts the quench rate to result in as rapid a quenching of the coke and drum as will be tolerated, without increasing the internal pressure of the drum above a maximum limit. The buildup of internal pressure in the drum is due to the vaporization of the quench water to form steam, which must be vented from the drum. This method usually results in an essentially constant drum internal pressure during the quenching procedure, and allows the quenching to occur in a short time period. The quench flow rate, in this method, may be adjusted manually by the operator, who monitors the coke drum internal pressure as indicated by the overhead pressure, to maximize the quench water flow rate.
Alternatively, as shown in U.S. Pat. No. 3,936,358 to James E. Little, an automatic control can be used to monitor the coke drum overhead pressure to maximize the quench water flow rate in response to the coke drum internal pressure. A substantially constant internal coke drum pressure is maintained. In another method, as shown in U.S. Pat. No. 4,358,343 to Franz Goedde et al., the quench rate is varied with time to maintain the vapor pressure decay rate, above the coke bed, in accordance with an ideal curve.
These previous methods often rely on periodic routine inspection and maintenance of the coke drum to detect and repair damage resulting from the accumulated effect of high metallurgical stresses imposed on the drum. Although such inspections and maintenance are expensive and time consuming, the quench time is reduced.
Quenching the coke drum at a maximum or a constant high internal coke drum pressure, however, leads to an eventual accumulation of inelastic strain in the coke drum. When a hot coke drum is quenched, a ring of high thermal stress forms in the coke drum from the significant differences in drum wall temperature over a small vertical or longitudinal distance. This high temperature differential over a small vertical distance is referred to as a longitudinal thermal gradient. These significant longitudinal thermal gradients are associated with a water level that rises through the coke drum.
A high longitudinal thermal gradient and an excessive internal coke drum pressure are the major contributors to the formation of stresses in the coke drum. Over a period of many coking cycles, the accumulation of inelastic strain and stress, in the metal of the coke drum, results in the metal bulging, cracking and thinning. This ultimately acts to decrease the lifetime of the coke drum.